The majority of research on the etiology of autoimmune diseases is focused on dysregulation of T-cell and B-cell tolerance mechanisms as the primary causes of these diseases. However, there is a growing realization that innate immune cells can be major contributors to autoimmunity. Mice lacking the Src-family kinase Lyn, which is expressed in B-lympocytes and innate immune cells, develop a profound autoimmune syndrome that resembles human systemic lupus erythematosus. These mice produce a number of autoreactive Abs, including anti-dsDNA, anti-RNA and anti-nuclear Abs that lead to immune complex deposition in the kidneys and progressive glomerulonephritis. A number of studies have focused on alteration in B-cell signaling thresholds, leading to abnormal B-cell selection, as the mechanism of autoimmunity in /yn"x" mice. Lyn deficiency causes severe dysregulation in the myeloid cell compartment as well, characterized by hyperproliferative responses and increased cytokine secretion following stimulation with a number of agonists. By generating a series of mixed chimeric mice containing only \yri''myeloid cells, but WT B-cells and T-cells, we have found that the hyperreactive Lyn-deificient myeloid cells alone are sufficient to drive selection of WT autoreactive B-cells, leading to production of anti-dsDNA, anti-RNA and ANA autoAbs resulting in immune complex deposition in the kidneys. Interestingly, both regular /yn"A mice and the lyn''myeloid cell chimeras have very high levels of serum BAFF, a cytokine produced by innate immune cells that has been implicated in autoimmunity in both mouse and man. The central hypothesis of this application is that overproduction of BAFF by Lyn deficient innate immune cells, in response to endogenous TLR ligands or interferons, plays a major role in the autoimmune process in this model. We will test this hypothesis in three specific aims: 1) analysis of macrophage, dendritic cell and neutrophil lineage specific mutations of Lyn kinase, using a newly generated lyn conditional knockout mouse, to define which innate immune cell lineage is responsible for driving autoimmunity;2) use of lentiviral or mAb mediated means to reduce serum BAFF levels and ask whether this will amerliorate autoimmunity in \yri'~ or \yri'~ myeloid chimeric mice;3) determine whether chronic TLR stimulation, leading to elevated production of IFNy may be responsible for driving hyperactive Lyn-deficient myeloid cells to produce BAFF, thus exacerbating autoimmunity. Our project will depend on collaborations with other projects in this PO1, both to provide us reagents and insight to investigate autoimmune mechanisms in lyn mice as well as to investigate potential aspects of autoimmunity in other models within the group. (LAY): Autoimmune diseases such as rheumatoid arthritis and lupus are caused by multiple immune system defects. This project will focus on the idea that genetic mutations that affect immune cells adapated to fight infectious pathogens, referred to as the innate immune system, can also lead to autoimmune disease in a mouse model. If this hypothesis is validated, it will significantly alter our understanding of autoimmunity in human patients.